The Air Force reveals training, tests and other changes made in response to concerns about oxygen systems on its F-22 Raptor fighter jet.

In response to growing concern about problems with its F-22 Raptor fighter jet, the Air Force revealed it has slapped on new safety restrictions to protect its pilots.

The announcement came as Sen. Mark R. Warner (D-Va.) and Rep. Adam Kinzinger (R-Ill.) Friday requested additional information from the secretary of the Air Force to further determine the scope of safety concerns raised by several pilots of the world's most expensive fighter jet, designed and built byLockheed Martin Corp.

The Air Force acknowledged last week that some of the nation's top aviators are refusing to fly the radar-evading F-22, a fighter jet with ongoing problems with its oxygen systems that have plagued the fleet for four years.

"The health and safety of our pilots — all of our pilots — is the utmost priority," said Brig. Gen. Daniel O. Wyman, an Air Force command surgeon. "Our operational flight surgeons and medical staff interact with our pilots on a daily basis, and mission No. 1 is their health and safety."

The comments, posted on the Air Force's website, were meant to address the growing attention directed at the safety of the F-22. Concerns have grown in recent months as no clear explanations have emerged for why pilots are reporting hypoxia-like symptoms in the air. Hypoxia is a condition that can bring on nausea, headaches, fatigue or blackouts when the body is deprived of oxygen.

The F-22 is considered the most advanced fighter jet in the world. It entered military service in 2005, and the Air Force received the last of its order of 188 planes last week.

The plane can reach supersonic speeds without using afterburners, enabling it to fly faster and farther. It's also packed with cutting-edge radar and sensors, enabling a pilot to identify, track and shoot an enemy aircraft before that craft can detect the F-22. The Air Force says the aircraft is essential to maintain air dominance around the world.

According to the Air Force, each of the sleek, diamond-winged aircraft costs $143 million. Counting upgrades and research and development costs, the U.S. Government Accountability Office estimates each F-22 costs taxpayers $412 million.

While other warplanes in the U.S. arsenal have been used to pummel targets in Iraq, Afghanistan and Libya, the Air Force's F-22s have sat largely idle — used only in test missions. Even so, throughout the jet's development, F-22 pilots have been in seven serious crashes, resulting in two fatalities.

Over the years, F-22 pilots have reported dozens of incidents in which the jet's systems weren't feeding them enough oxygen, causing wooziness. This issue led to the grounding of the entire F-22 fleet last year for nearly five months. But even after the grounding was lifted, the Air Force said investigators could not find a smoking gun.

The Air Force lifted the grounding last September. When that happened, Wyman revealed this week, the Air Force put all F-22 pilots through retraining so they would know their own specific hypoxia symptoms. It also affixed a device to pilots' fingers that measures the amount of oxygen in the blood while they are in the cockpit.

The Air Force also added a high-efficiency particulate air filter consisting of activated carbon and charcoal, Wyman said. "It was cleared for flight use by theU.S. Air Forceprogram office and has been used by the military for over a decade in the ground crew and aircrew ensembles," he said.

At the end of each flight, pilots turned in the filters to be examined by Air Force personnel.

Black dust was found in some of the breathing hoses.

"We analyzed it and found it to be activated carbon dust ... an inert or nonreactive compound that has been used for air and water filtration for decades without any significant evidence of harm," Wyman said. The dust was "well below the industrial hygiene standard levels set by government agencies," he said.

In addition, Wyman revealed, the Air Force conducted throat swabs of F-22 pilots, and those indicated no evidence of harmful substances. Even so, pilots reported persistent coughing, which they call the "Raptor cough."

"Coughing is a natural physiologic response that serves to re-inflate the air sacs," Wyman said.

Last Sunday, two F-22 pilots appeared with Rep. Kinzinger onCBS'"60 Minutes" to discuss reasons why they refused to fly the jet. At the risk of significant reprimand — or even discharge from the Air Force — Virginia Air National Guard Capt. Joshua Wilson and Maj. Jeremy Gordon said they would not fly the F-22 until the oxygen problems were solved.

Since the segment aired, other pilots have contacted Sen. Warner of Virginia, which is home to one of the seven military bases where F-22s are based.

"After meeting with these pilots, and having conversations with many other knowledgeable individuals, we would recommend an immediate, confidential and anonymous safety survey of all active duty and reservist F-22 crews, pilots and flight surgeons to definitively document the scope and frequency of these hypoxia-like incidents," Warner and Kinzinger wrote in a letter to Air Force Secretary Michael B. Donley. "It is our view that such a survey could be initiated within 10 days, and our offices would expect to receive timely updates both on the survey methodology and the results shortly thereafter."

The Air Force did not reveal how many of its 200 F-22 pilots had declined to fly the jet.